The vlantransparent extension's API definition was rehomed into
neutron-lib with commit I78c3e0c0b74dd154b6133963dfc8b65f9527bd2c
This patch consumes it by using neutron-lib's implementation in prep
for Ibfaa1ebf24caec62f5743975b206400fcd30436d
Change-Id: I8e2bf3fb75b3ccbcb269677fb2aa826659fbbdd2